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Earth-time is VASTLY huge, and it is broken down for convenience into a hierarchy of divisions: the largest divisions are called Aeons: these are each divided into Eras, each in turn divided into Periods, and each of these divided into Epochs. These four levels are shown below. Not all of the Periods of the paleozoic are named; only the Teriary and Quaternary Epochs are shown.
The Hadean starts at earth's beginnings, some 4,500 million years ago (m.y.a.); life begins in the Archaean; it develops in the Proterozoic; it expands enormously in the Phanerozoic. This Phanerozoic Aeon is the one most of you will know something about, and it is divided into three Eras, the Paleozoic, the Mesozoic ("the Age of Reptiles") and the Caenozoic ("the Age of Mammals").
We all know that the earth is really old. But how old is that, exactly? What does 4,500 million years mean? How much older is the earth itself than the life it sustains? than eukaryote life? than animals? than mammals? than us?
Most people have a really hard time coming to grips with
time, especially when long periods are involved. We can readily understand
time periods in the order of a human generation, or a life- span;
our grasp of historical times is a bit shaky, but as soon as we step much
beyond that, time becomes progressively more difficult to conceive of and
comprehend.
You see the problem. It's very like the problem
of spatial scale: is the earth 10 million metres across? or
100 million? maybe a billion.......?
People just have a hard time comprehending the meaning of big numbers....... and tiny numbers too..... it's a problem of being aware of what difference an extra zero or two, or few, makes.
In science circles, a common way of dealing with things that are of vastly different magnitudes is to use a logarithmic scale, and a common approach is to use so-called powers of ten - each equal step up the log-scale involves a multiplier of ten in terms of the original units which have been "logged." This has the effect of bringing very large numbers down, and bringing very small numbers up, to where you can "see" them, better comprehend them.
Each step in the powers of ten log-scale takes us from one so-called "order of magnitude" to the next: 102, 103, 104, 105 etc.
As an attempt at making the hugely varying scale of evolutionary time more comprehensible and, one hopes, also more easy to recall and use, here you will find time scales as charts based in powers of ten.
The first time-chart below provides a road-map to total time: from the Recent (10,000 years ago - when human agriculture began) back to the Big Bang (when the whole universe began). The chart is an overall guide to the context in which the later charts are embedded. In this General Time Scale, we see the log-scale on the left compared with the conventional linear scale on the right. You can readily see how the log-scale crunches up the far distant times and stretches out the relatively recent past, as compared to the linear scale of time.
Except for the first of the following detailed charts, the charts all show "windows" of time which span, in each step, a single power of ten year: each spans a period ten times longer than the window preceding it. We begin at the familar, historical, end of time - only a thousand years ago - thirty human generations - progressing in ever-increasing steps, eventually reaching a very coarse-grained picture of the far, far distant past, even before the earth existed.
Although it perhaps makes some sense to "start at the beginning" and work forward in time from earth's origins to the present, I have chosen instead to start at the present and work back towards the past. This is because, taking into account the difficulty that most of us find in dealing with time, I think it may be easier to start out with the more familiar, recent, past, and then proceed to ever-more-distant times.
I think it also makes sense to do it this way, since we refer to these times from a vantage point of the here-and-now - our zero is the present, and our time-scale reads larger as we recede into the past.
But if you find it makes more sense to go the other way, the choice is yours............... just look at the charts in reverse order.
Chart One shows more or less familiar things, and, unlike all the following charts, it takes us through three powers of ten, from 1,000 (103) years ago to 1,000,000 (106) years ago - roughly the span of mankind.
Apart from the timing of the Pleistocene glaciations and associated events, most of the matters mentioned in this chart aren't the subject of this course: they are the stuff of anthropology, archaeology or history.
Chart Two takes us from 1 million to 10 million years back: it's the time during which modern earth climates finally develop fully, the genus Homo appeared.
Chart Three takes us from 10 million to 100 million years ago - a giant step, for this window takes us from near the threshold of mankind back to the times when the dinosaurs dominated the world. In going back through the time shown in this window, global climate changes from the cooler and dryer conditions which typify the late Cænozoic to the nearly-uniform warm & humid conditions typical of most of the Palæozoic and Mesozoic.
The next step (Chart Four), is truly enormous. It includes most of the Phanerozoic Æon, where atmospheric oxygen approaches and eventually reaches (during the Devonian) modern levels, and multicellular organisms diversify into familiar forms, including dinosaurs, flowering plants, and the first birds. But it begins one billion years ago, at the top of the Proterozoic Æon, when the atmosphere was very different from now, and most life was still bacterial.
Chart Five shows us the vast, dizzying expanses of the first three-quarters of all of earth's history, when life began, and beyond - to 10 billion years ago, before the formation of this planet, or the solar system, or even this galaxy.
Much of this window is occupied by the Archaean and Proterozoic Æons, a time before eukaryotes, when bacteria ruled the earth under a strange atmosphere, and virtually all life was in the oceans. At the top of the window, global photosynthesis by cyanobacteria has just begun to add significant amounts of oxygen to our atmosphere, and eukaryotes appear.
So: seven powers of ten, in year units, take us from the times of Attila the Hun back to the primæval galactic gas clouds before the sun and earth and other planets formed.
Remember: each chart fits in one tenth of the time shown on the chart following it; each chart shows 10 times more time than the one preceding it.
It is my hope that, by showing time in this fashion, in a few windows bounded by simple numbers (just successive powers of ten), you will be able to grasp more firmly the meaning of time, and therefore be better able to recognise the location of important elements in the history of earth's biota. I also hope that the ordering and meaning of the standard divisions of the geological column (which refer only to the Phanerozoic Æon) will prove easier to recall and to use.
© Paul Handford 2000